* Potential Energy: While the book is sitting on the shelf, it has *potential energy*. This is stored energy due to its position relative to the ground. The higher the shelf, the more potential energy the book has.
* Kinetic Energy: As the book begins to fall, it starts gaining speed. This means it's gaining *kinetic energy*. Kinetic energy is the energy of motion.
So, as the book falls, it loses potential energy and gains kinetic energy. The total energy (potential + kinetic) remains constant, following the law of conservation of energy.
